1. Field of the Invention
The present invention relates to a motor control unit which drives a motor using a power converting semi-conductor, and more particularly to the overheat protection of the power converting semi-conductor and the temperature control of the motor control unit.
2. Description of the Prior Art
A first example of a conventional motor control unit provided therein with overheat protection function is shown in FIG. 14. In the first example as shown in FIG. 14, a three-phase alternating current motor is used as a motor and an inverter is used as a power converter, respectively.
In FIG. 14, reference numeral 1A is a motor control unit, reference numeral 2 is a motor, 3A a control computing unit and 4A a power converting semi-conductor. The power converting semi-conductor 4A is provided with three-phase switching arms (U phase arm, V phase arm and W phase arm), an overheat identification means 8A and an overheat protection means 9A. The U phase arm that is one of the switching arms consists of an upper arm switching element 5a, a lower arm switching element 5b, an upper arm freewheeling element 6a, a lower arm freewheeling element 6b, an upper arm gate switching element 7a, a lower arm gate switching element 7b and thermistors 10 and 11. The V phase arm and the W phase arm have the same structure as the U phase arm. A bipolar transistor is used here as the switching element. In the drawing, C, G and E show a collector, a gate and an emitter, respectively.
The motor control unit 1A generally converts direct-current power from an electric power unit (not shown) into alternating-current power and supplies it to the motor 2. The conversion from direct-current power to alternating-current power is carried out by switching the switching elements consisting of a power element of the power converting semi-conductor 4A. The power elements consist of the switching elements and the freewheeling elements. A gate driving signal which is generated in the control computing unit 3A for switching is connected to gates G of the switching elements through the gate switching elements. When the switching elements are electrified by switching, they generate heat due to internal loss. Overheating may destroy the switching elements. To avoid the possible destruction of the switching elements, overheat protection is provided by monitoring the temperature of the switching elements, cutting off (gate cut) the signal flowing from the control computing unit 3A to the gates of the switching elements when the temperature reaches a predetermined level, and breaking the electric current sent to the switching elements. A protection function operation alarm signal is transmitted to the control computing unit 3A when the overheat protection is carried out. In the drawing, an overheat identification means 8A determines by a signal from the thermisitor 10 whether the temperature sensing and gate-cut are necessary or not. The signal for gate cutting is generated in the overheat protection means 9A to effect the gate cutting by gate switching elements 7a and 7b. The thermistor 10 is disposed near the switching elements to reflect the temperature of the switching elements correctly.
Another thermistor 11 is disposed on the periphery of the switching elements as a means of informing the control computing unit 3A of the temperature of the switching elements. In the control computing unit 3A, the temperature information of the switching elements is obtained from the thermistor 11 by, for example, a microcomputer or an A/D converter. With this temperature information (signal), a gate driving signal is operated so that the temperature of the switching elements us not raised excessively.
As a second conventional example, Japanese laid-Open Patent Application (Kokai) No. Hei 10-21079 discloses a control unit for protecting switching elements from overheating by using a temperature signal from a power converting semi-conductor. In this second conventional example, a motor control unit for proving overheat protection is applied to an electric vehicle. FIG. 15 shows this second conventional example.
In FIG. 15, reference numeral 2 is a motor, reference numeral 12 is a battery, 13 a power converter, 14 wheels, 15 a power converter ECU (an electronic control unit), 16 an accelerator pedal, and 17 a temperature sensor, respectively.
The power converter 13 is connected to the battery 12 serving as a power unit and the motor 2 for driving the vehicle is connected to the power converter 13. The driving force of the motor 2 is transmitted to the wheels 14 through a rotation axis and a differential gear to serve as the propulsive force of the vehicle. The power converter 13 is provided with the power converting semi-conductor. Also, the power converter 13 is controlled by the power converter ECU 15, and the power converting semi-conductor within the power converter 13 starts a switching operation according to a gate driving signal input from the power converter ECU 15. With this switching operation, the direct-current power supplied from the battery 12 is converted to alternating-current power to be supplied to the motor 2.
The power converter ECU 15 is connected to an accelerator pedal 16 and adapted to detect an amount of pressure as an accelerator opening A% when a driver depresses the accelerator pedal 16. It is however noted that the accelerator opening is 100% when the accelerator pedal is fully pressed.
The power converter ECU 15 is also connected to the temperature sensor 17 disposed within a case of the power converter 13 to gain the temperature INV-T of the switching elements. Further, the power converter ECU 15 determines the amount of change of the element temperature per unit time based on the element temperature INV-T to allow the temperature change rate to be .DELTA.T/.DELTA.t.
The power converter ECU 15 determines a torque to be output from the motor 2 according to the accelerator opening A% and converts the torque found to a torque command*. Further, the following two kinds of limiting rates (a first limiting rate .alpha., a second limiting rate .beta.) multiplied by the torque command T make the regulated torque command T*. This first limiting rate .alpha. is determined based on the element temperature INV-T. The first limiting rate .alpha. is 100% when the element temperature INV-T is below a limitation start temperature T1. When the element temperature INV-T is higher than the limitation start temperature T1, the first limiting rate .alpha. corresponding to the element temperature INV-T at that time is multiplied by the torque command T*. When the element temperature INV-T reaches a zero-power temperature T2, the first limiting rate .alpha. is 0 and the torque command T* is also 0.
The second limiting rate .beta. is determined based on the temperature change rate .DELTA.T/.DELTA.t. The second limiting rate is used when the element temperature INV-T is above the limitation start temperature T1. When the temperature change rate .DELTA.T/.DELTA.t is below a first reference value .delta.1, the second limiting rate .alpha. is 100%. When the temperature change rate .DELTA.T/.DELTA.t is higher than the first reference value .delta.1, the second limiting rate .beta. corresponding to the temperature change rate .DELTA.T/.DELTA.t at that time is multiplied by the torque command T*. When the temperature change rate .DELTA.T/ .DELTA.t is above a second reference .delta.2, the second limiting rate .beta. is 0 and the torque command T* is also The power converter ECU 15 obeys the regulated torque command T* found by the above and a command value I* of a motor electrifying current corresponding to the regulated torque command T* . The generating torque of the motor 2 is controlled to coincide with the regulated torque command T* by switching the switching elements of the power converting semi-conductor based on this current command value I*.
As described above, the switching element temperature is high in the second conventional example and when it rises quickly, the switching elements are kept from overheating by regulating the torque command substantially. On the other hand, when the temperature of the switching elements is high but has not risen quickly, the torque command need not be regulated as much as in the quick rise. Thus, it is possible to prevent the switching elements from overheating while regulating the torque according to the extent of the temperature.
According to the first conventional example, it is possible to provide overheat protection by switching stop (gate cut) and overheat prevention by temperature rise monitoring relative to the temperature rise of the switching elements in the power converting semi-conductor.
However, according to the first conventional example, the thermistor 10 serving as the temperature sensor for overheat protection differs from the thermistor 11 serving as the temperature sensor that transmits the temperature of the switching elements to the control computing unit 3A. It is also difficult to let the temperature of the switching elements indicated by the thermistor 10 coincide precisely with that of the switching elements indicated by the thermistor 11 due to the distance of the switching elements from their tips as seen in the arrangement and the dispersion of electrical characteristics of each sensor. Therefore, when it is required to operate a gating signal for switching the power converting semi-conductor 4A for overheat prevention through temperature monitoring by the thermistor 11 before the overheat protection by the thermistor 10 is activated, it is necessary to absorb the dispersion of the sensor by setting a threshold temperature starting operation of the gating signal to a lower value. However, when the threshold temperature is thus set to the lower value, there is some possibility that the overheat prevention process is frequently carried out even in a normal operation and this is therefore not practical. On the contrary, when the threshold temperature is set to a normal value (not low), there is some possibility that the overheat protection by the thermistor 10 is carried out prior to the start of the overheat prevention process depending on the dispersion of the sensor, and the switching operation of the switching elements stops to interrupt the control. Such a property is not desirable in the motor control unit of the present invention.
Also, according to the second conventional example, even when the switching element temperature of the power converting semi-conductor rises suddenly, it is possible to prevent the switching element from overheating by regulating not only a torque command of the motor, but also an electrifying current command according to a rate of change per unit time of the switching element temperature. The regulating amount of the torque command is increased or decreased to provide the proper torque regulation according to whether the switching element temperature is high or low.
However, in the second conventional example, when a temperature signal of the switching element to be connected to the power converter ECU 15 changes suddenly for some reason such as wire breakage or the temperature sensor failure of the switching element, the torque command is regulated substantially because a rate of change per unit time of the switching element temperature is large, thereby resulting in a sudden change in the generating torque of the motor. This property is not desirable in an on-vehicle motor control unit including an electric vehicle because it has a bad influence on the continuity of control and affects the vehicle behavior system as well.